Tubular door locks generally have different functions and safety standards according to their uses. For example, one used in a door to enter a house should have high safety against burglary, but ones used in a door of a bathroom, a toilet or a bedroom etc. in a house do not need high safe locking structure. Nevertheless, they are always made of metal, to a resultant high cost in manufacture. A U.S. patent application Ser. No. 07/735,263, filed Jul. 24, 1991, now U.S. Pat. No. 5,157,952, issued Oct. 27, 1992 and titled "Tubular Door Lock" by this same applicant offered a kind of structure, wherein locking the lock by an inner knob is accomplished by first pressing and subsequent rotating of said knob, and unlocking by said knob can be done only by rotating of said knob.